I have just installed an Icom ID-5100A in my old Landcruiser and am now having a strange issue that I am hoping someone might be able to help me with.

The vehicle has a REDARC SBi12 dual battery isolator installed with a (Blue) wire running from the solenoid into the car for remote operation should the start battery go flat, which connects both batteries together and enables the car to then start off the aux battery.

My problem is when i transmit on 70cm at 50w, the solenoid cycles on and off rapidly until the transmission stops.
If i turn down the power output of the radio to 25w, no more problem.
If i transmit on 2m at 50w, no problem.
Problem only occurs on 70cm at full power (50w).

I have tried installing a toroid ferrite next to the solenoid on the remote control wire (Blue) with a little bit of success.
The cycling of the solenoid has now stopped, however it turns on and stays on while ever transmitting with this toroid in place.
I'm assuming that I need to bleed off the RF that is being induced into this wire somehow.

I have attached a picture of how the solenoid is setup up in my car (Option 2).

One other thing that might be worth a try is installing a ceramic capacitor from ground to the relay line at the unit input.
Experiment with values if needed but you might start with a 0.1 mf and test the effect.
luck,
Doug